The state of the art pertaining to filtration of liquids through a bed of granular filter media has progressed considerably over the past fifteen years. The concept of multimedia deep bed filters (or in-depth filters) in a well known art. Also it is commonly known that arranging layers of granular media from the coarsest to the finest produces the most efficient filter operation. Filtering liquids through a series of deep bed filter layers has been disclosed un U.S. Pat. Nos. 3,171,801, 3,171,802, 3,701,423 and 3,680,701. A multiplicity of filter media combinations has been suggested, for example in U.S. Pat. No. 3,343,680, as including combination of anthracite, sand, garnet, plastics, and activated carbon.
In spite of all the advancements that have been made, the prior art does not set forth solutions to many of the more troublesome problems of liquid filtration, particularly those associated with filtration of industrial and municipal waste water. For instance, it is commonly known that most any suspended contaminant can be removed from a liquid by filtering through a bed of granular filter media, provided the granules are fine enough and the bed is deep enough. However, a high pressure drop across the filter bed and frequent backwashings often make such "super" filters economically impracticable.
Filters are also often subjected to emergency or peak load conditions. Without providing for an overdesigned filter or auxiliary filter to handle peak loads, deep bed filters of the prior art simply cannot contend with most emergency or peak load conditions. In many municipal and industrial waste treatment facilities upset conditions may occur from time to time. Suspended solids carried to a filter during an upset condition will be so great so to often render a deep bed filter totally inoperable.
Attempting to solve these prior art problems by simply employing a filter bed having a number of filter layers creates even more problems. For example, practical considerations foil attempts to design filters which meet the large to fine granule gradation. Using material of the same specific gravity will produce a reverse fine-to-large gradation after the filter bed has been backwashed. Up-flow filters are designed to circumvent this problem, but these filters have definite limitations as to minimum size of finer filter media and as to overall flow rate.
The reverse gradation problem has been solved with a certain degree of success by using materials of differing specific gravities. The upper and coarser filter media is comprised of a lighter weight material such as anthracite coal. The successive layers are comprised of increasingly more dense materials. An example of such a combination would be a top layer of anthracite, an intermediate layer of sand and a bottom layer of garnet. This and other combinations are discussed in U.S. Pat. No. 3,343,680.
However, with the use of material of differing specific gravities there is a practical limitation on the size of the largest filter media that can be used. Even with materials of such different specific gravities as coal and sand, if the granules of coal are large enough they will tend to stratify at lower levels within a multimedia filter bed. And it is such larger granules that are needed to handle the conditions of heavy dirt loads with peak or upset conditions at waste treatment plants.
The device shown in U.S. Pat. No. 3,680,701 attempts to solve this problem by separating the larger and finer filter media into separate beds. However, simply separating the various grades of filter media into individual beds is not the answer to handling varying filtration conditions.
The present inventIon overcomes the previously stated problems of prior art devices by the provision of a plurality of physically separated filtration vessels which are interconnected by fluid flow conduits. Each vessel houses either one or a plurality of layers of granular filter media in an arrangement such that the granular media becomes finer in the direction of filtration flow. Further, each vessel is designed to successively reduce filtration flow rate. With such an arrangement, improved filtration of liquids is accomplished, varying contaminant loads can be accommodated, and reverse stratification of filter media during backwash procedures is substantially eliminated.